Field of the Invention
The present invention relates to an exhaust system connected to a power unit that burns fuel to generate power while generating exhaust gas.
Description of the Related Art
Along with a recent rise in the awareness of environmental issues by the society as a whole, various techniques have been developed in the technical field (e.g., automobile engines) of generating power by burning fuel so as to remove harmful components such as nitrogen oxides from exhaust gas that is generated during burning of the fuel. Such removal of harmful components from exhaust gas is typically performed through a chemical reaction (e.g., reduction or oxidation) of the harmful components caused by catalyst so as to convert the harmful components into a relatively not-harmful other components. Many of such catalyst have a property of activating its removal function for the harmful components by being heated. It is the simplest way to use, as a heat source to heat the catalyst, exhaust gas flowing toward the catalyst for the removal treatment.
Meanwhile the latest power units have improved power generation efficiency (power energy generated from a unit of fuel) because of the influences of increasing demands for saving of energy (e.g., saving resources), and so the amount of heat of exhaust gas generated from a unit of fuel in such power units tends to decrease. This requires some method to heat the exhaust gas flowing toward the catalyst so as to keep the exhaust gas at high temperatures and accordingly keep the active state of the catalyst.
One of the methods of increasing the temperature of exhaust gas may be of simply increasing the amount of fuel per one cycle used to generate a predetermined amount of power energy through the burning of the fuel, so as to increase the amount of heat generated in addition to the power energy. This method, however, basically requires continuing the increased state of fuel per one cycle so as to keep the active state of catalyst. Since this adversely affects the power generation efficiency greatly, this method is not practical. As another method to keep exhaust gas at high temperatures, it is proposed that exhaust gas flowing toward catalyst for a removal treatment is heated by a heater including a honeycomb structure with electrode (see Patent Document 1, for example).
[Patent Document 1] JP-B-2931362